The Importance of Shear Sensitivity in Pump Selection
Shear thickening
Viscosity
Newtonian liquids (“ normal ”)
Shear thinning
Stress ( application of force )
Are you handling or transferring a shear sensitive fluid ? Does your manufacturing process involve a liquid that alters when heavy force like stirring is applied ? This article from Castle Pumps Ltd will give you more of an insight into exactly what a shear sensitive product is and how this impacts pump selection . So , let ’ s start with a deeper look at the characteristics of a shear sensitive liquid …
What are shear sensitive fluids ?
Shear sensitive liquids are those that become altered or even damaged when force is applied . In the most simplistic of terms , they are delicate fluids that change viscosity when “ messed with ” like shampoo turning into foam , or that contains solids that can be crushed but for the purpose of their use should not be , like fruit pieces in a yoghurt .
In more scientific of explanations , all liquids can be categorised into either Newtonian or non- Newtonian . Newtonian fluids are those that remain at a constant viscosity no matter the amount of shear applied ( providing temperature remains the same ). These include water , alcohol and mineral oil .
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With non-Newtonian fluids , how thick or thin the liquid is can alter depending on the composition of the product and how it reacts to shear being applied . The way the liquid reacts can be identified in four ways - Dilatant ( Shear Thickening ), Pseudoplastic ( Shear Thinning ), Rheopectic and Thixotropic .
• Dilant - become much thicker when force is applied - think of a cornflour and water mixture or pancake batter .
• Pseudoplastic - are the exact opposite , with them become less viscous with increased stress - just like tomato sauce !
• Rheopectric – become more viscous as more stress is applied over time – if you whip cream for long enough , it becomes much thicker .
• Thixotropic – becomes less viscous as more stress is applied over time – if you stir honey over a period of time it becomes much more fluid .
But why does this impact pump selection and what exactly is a shear sensitive pump ?
What is a low shear pump ?
Whilst certain pumping technologies such as centrifugal pumps apply high shear in operation , shear sensitive pump designs are able to keep such fluids intact due to their low internal velocity , gentle pumping mechanism and the wider tolerances between their pump casing and internal components . Failing to select a low shear pump when handling a shear sensitive liquid can result in product inconsistencies , damaged solids that need to be intact and costly waste .
Typical low shear pump types are typically positive displacement pumps and include peristaltic pumps , progressive cavity pumps , flexible impeller pumps and AODD pumps .
Does your process involve the handling of a shear sensitive fluid that you cannot risk altering or damaging ? Contact the pump experts at Castle Pumps Ltd for help specifying the right pumping solution .
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